1. Field of the Invention
This invention generally relates to the modification of mechanical refrigeration or air conditioning systems to include a complete, integrated liquid refrigerant pumping system. Applicant's system also proactively protects the refrigerant pump from damage related to cavitation and the loss of lubrication as result of vaporous refrigerant entering the pump, by consistently supplying the pump with liquid refrigerant above its NPSH requirement. Applicant's system also contains itself within a greatly reduced footprint allowing the system to be installed or retrofitted in spatially limited areas.
2. Description of the Prior Art
In the operation of commercial freezers, refrigerators, air conditioners, and other compression-type refrigeration systems, is desirable to maximize refrigeration capacity while minimizing energy consumption. One means to reach that goal has been to include a liquid refrigerant pump in the refrigeration system circuit in order to suppress the formation of “flash gas”. Flash gas is the spontaneous flashing or boiling of liquid refrigerant resulting from pressure losses in the liquid refrigerant conduit. The prior art identified that it is beneficial to include a liquid refrigerant pump in the liquid refrigerant conduit that terminates at the refrigerant expansion device in order to suppress flash gas in that conduit.
As an example of these prior attempts, see U.S. Pat. Nos. 4,599,873; 5,150,580; 5,386,700; 5,626,025; 5,752,390; 6,145,332 all issued to Hyde, and U.S. Pat. No. 5,749,237 to Sandofsky.
Additionally, the prior art identified the disastrous effect that flash gas has on the liquid refrigerant pump if it is allowed to enter the pump through the pump inlet. Excess flash gas entering the liquid refrigerant pump will cause cavitation; since vaporous refrigerant cannot be returned to its liquid state by means of a liquid refrigerant pump, there will consequently be no hope of suppressing flash gas in the conduit leading to the refrigerant expansion device by means of said liquid refrigerant pump. Additionally, cavitation causes pump damage.
An example of a prior art attempt to address the problem of flash gas entering the liquid refrigerant pump through its inlet is U.S. Pat. No. 6,145,332 issued to Hyde. The patent delineates a means to protect the liquid refrigerant pump from cavitation due to vaporous refrigerant entering the pump through its inlet by means of a reactive approach.
Past attempts to implement the prior art have not always supplied the liquid refrigerant pump with its required NPSH. Space constraints often do not permit a vertical column of vapor free liquid refrigerant that is high enough above the center line of the liquid refrigerant pump inlet to insure that the required NPSH is available to the liquid refrigerant pump.
The need remains to address the problem of flash gas entering the liquid refrigerant pump through the pump inlet.
Applicant's invention protects the liquid refrigerant pump from cavitation due to vaporous refrigerant entering the pump through its inlet by means of a proactive approach. Applicant's invention provides for consistent net positive suction head or NPSH above that required by the pump manufacturer at the pump inlet to minimize the likelihood of the occurrence of “flash gas” and cavitation. NPSH refers to the amount by which the liquid pressure at the pump inlet exceeds the saturation pressure of the liquid. The need to provide for sufficient NPSH to prevent cavitation is essential in the design of refrigeration or air conditioning systems which include liquid refrigerant pumps.
NPSH available to the liquid refrigerant pump is calculated by measuring the height of the vapor free column of liquid refrigerant that is above the centerline of the pump inlet and subtracting all pressure losses as it travels from there to the centerline of the pump inlet. The positive pressure difference between the liquid pressure at the centerline of the pump inlet and the liquid pressure at the top of the vapor free column of liquid refrigerant is correlated to the NPSH available. Due to the weight of the liquid refrigerant, NPSH available increases as the height of the liquid refrigerant column increases.
NPSH required by the liquid refrigerant pump is supplied by the pump manufacturer. NPSH required is the minimum height of liquid above the pump inlet that will allow the pump to operate without cavitation caused by vaporous refrigerant at the pump inlet.
Applicant's system prevents “flash gas” at both the inlet and outlet side of the liquid refrigerant pump by maintaining vapor free liquid refrigerant in the portion of the system loop entering the liquid refrigerant pump at a sufficient height above the pump inlet to meet or exceed the pump's required minimum NPSH. Applicant's system does this automatically without an excess amount of control technology, utilizing the pressure differentials of the system and gravity.